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The enormous viscosity of the early universe prevented both turbulence and gravitational structure formation. The universe cooled after the big bang, and rest mass dominated energy/c^2 after about 10,000 years. Soon after that, viscous forces decreased below inertial forces and gravitational forces: 30,000 years based on the superclustervoid size taken to be a fossil of the first fragmentation. The non-baryonic dark matter (possibly neutrinos) diffused out of the fragments into the voids until the diffusive Schwarz scale L_SD = (D^2 / \rho G)^1/4 became less than the expanding void size, so that this weakly collisional material could become gravitationally bound as superclusterhalos, clusterhalos and outer galaxy halos. From the measured sizes of these halos one can infer D values of order 10^28 m^2 s^-1, which is much larger than baryonic (ordinary) matter diffusivities. Turbulence was inhibited by viscous and buoyancy forces (except at the cores of protogalaxies and protoglobular clusters) until the first stars began forming from accreting PFPs at a few million years. The Jeans mass is irrelevant to structure formation except in protogalaxies at the plasma-gas transition point. At this time protoglobularclusters fragment at the Jeans mass because the maximum velocity of void formation is the sonic velocity, causing cooling, radiative heating, and resulting overpressures that cause this fragmentation.
astro-ph/9904230

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